1. Field of the Invention
This invention relates generally to improved methods and apparatus for growing crystalline ceramic products. The invention is particularly useful for the production of fibers and rods although the invention may also be used to prepare ceramic products with other shapes such sheets and tubes. More specifically, the invention is concerned with the provision of improved methods and apparatus for rapidly growing void-free shaped sapphire products, preferably fibers and rods, by so-called Edge-defined, Film-fed Growth. In addition to enabling the production of void-free ceramic products, the invention provides improvements in the as-grown crystal quality of ceramic (e.g. sapphire and silicon) in the form of sheets, rods and tubes.
2. Description of Related Art
Crystalline materials such as sapphire have many applications that require a highly pure material that is uniform throughout, including freedom from voids. For example, a major use of sapphire fibers is for transmitting light. Good optical transmission requires void-free fibers.
Various methods have been developed to prepare crystalline ceramic fibers. One common way to prepare such crystalline products is the Edge-defined, Film-fed Growth method (EFG), a high temperature melt-solidification process which is also known as Stepanov crystal growth.
EFG is a well-known technique disclosed in numerous U.S. Patents such as U.S. Pat. No. 3,471,265 to Bailey and U.S. Pat. No. 3,591,348 to LaBelle, the contents of which are incorporated herein by reference. EFG is also described in a number of non-patent publications. See, for example, LaBelle, R E., Mlaysky, A. I., Growth of controlled profile crystals form the melt: Part 1—Sapphire filaments, Mat. Res. Bull. 6: 571-580 (1971); LaBelle, H. E., EFG, the invention and application to sapphire growth, J. Crystal Growth 50: 8-17 (1980). The process is also described in detail in other references (see e.g. Sapphire & Other Corundum Crystals, E. Dobrovinskaya, L. Lytvynov, and V. Pishchik, Folio Institute for Single Crystals, Ukraine 2002).
Broadly described, a typical EFG apparatus as used to make sapphire fibers comprises a crucible provided with suitable means for heating crystalline material, e.g. α-alumina, within the crucible so as to form a melt, with means for feeding the melt upwardly to a surface where the melt is pulled away, as it solidifies, to form fibers. Typically the means for feeding the melt to the pulling area comprises a centrally bored rod, referred to herein as a die, which is positioned within the crucible and extends vertically from near the base of the crucible upwardly through the height of the crucible so that its upper end terminates just above the top of the crucible. The die includes a narrow bore which runs axially the length of the die so as to provide a central opening at the top end of the die surrounded by a flat lateral surface. The lower end of the die is open to the interior of the crucible to receive molten material therefrom. In operation, molten material in the crucible enters at the lower end of the bore, rises through the bore by capillary action and exits from the open top end of the bore where it forms a pool or film of molten material over the flat top end of the die. Positioned directly above this pool or film is a seed material which can be moved vertically up or down. Initially the seed is moved downwardly to contact the melt on the top surface of the die and it is then gradually moved upwardly drawing the solidifying melt with it to form fiber or rod of the desired diameter.
In the conventional EFG system as used to make fibers, the sidewall of the portion of the die which extends above the crucible and underlies the flat horizontal top surface that holds the melt, is at right angles to the flat top surface. Thus, in essence, the die for fiber production, generally comprises an axially bored right cylinder which is open at the top to permit melt to flow out of the bore onto the top end of the die where it is drawn away as it solidifies.
Generally speaking, the flat top surface of the die, hereinafter referred to as the die tip, is about 0.005 inches to 0.015 inches high, i.e. the die tip extends above the top of the crucible by this distance. The tip itself is about 0.002 inches to 0.030 inches across with the sidewall of the die in the conventional EFG system being at a right angle to the top surface of the die.
As noted, the die tip is wetted with molten film or pool of the source material supplied to the tip by capillary flow from the crucible via the bore of the die member. The growth of the desired crystalline body (e.g., sapphire fiber) is initiated by supplying seed crystal to the molten film at the die tip. Subsequently, the seed crystal is drawn upwards away from the tip of the die. A molten film of source material remains sandwiched between the growing crystalline body and the die tip. Since liquid material is continuously replenished via the crucible and capillary-forming die, continuous crystalline bodies such as e.g., fibers, rods, ribbons and tubes may be grown from the melt. In the production of sapphire fibers, the growing fiber is pulled steadily upwards, while the bottom of the fiber grows downward at the same rate where molten sapphire is solidifying at the bottom end of the fiber product. The molten material is replenished via capillary action through the central bore of the die to the die tip where the liquid material spreads out over the top surface of the die tip. At steady-state conditions, the process may be operated continuously and very long single-crystal fibers, for example, may be grown.
While the EFG process is generally effective to produce useful fibers and related shapes such as rods, it has been found difficult to run the process at relatively fast growth rates while maintaining void-free product. Voids tend to form just under the surface of the crystalline material during the EFG growth process as the material is drawn away from the tip of the die. These voids are thought to form because the fiber growing from a conventional die tip is cooler at the outer edge of the tip than in its center. This temperature difference causes the outside of the fiber to cool below the anneal temperature first, creating a relatively unyielding shell of crystalline substance on the surface of the growing fiber. As the solid material inside the fiber subsequently cools towards the anneal temperature, it contracts but since the outside of the fiber is already essentially an unyielding shell, shrinkage of the inside material leaves tiny voids in the crystal. These voids, which form in solid solution several fiber diameters above the die tip, greatly degrade optical transmission of the resulting product.
Fast growth of void-free optical fiber has been demonstrated on an experimental basis (Pollock, J. T. A., Filamentary sapphire, Part 3: the growth of void-free sapphire filaments at rates up to 3.0 cm/min, Journal of Materials Science 7: 787-792 (1972)). This method relied on heat shields placed with extreme precision. However, while void formation seems to have been reduced using this method, it has been found extremely difficult to reproduce the results and the method is not appropriate for commercial production.
Sapphire fiber for optical applications is conventionally grown very slowly to prevent void formation. Rates for commercial growth are reportedly at speeds of about 3 in./hr to about 6 in./hr. This slow speed reduces the heat of fusion rejected from the freezing sapphire and allows the temperature across the die tip to remain sufficiently uniform so that shrinkage voids are not created inside the growing sapphire fiber. However, due to the slow speed at which the sapphire fiber must be grown if voids are to be avoided, output is limited and the resulting fibers are, as a consequence, expensive. There is, therefore, a real need to provide methods and apparatus for growing void-free sapphire fibers and the like faster and at reduced cost. Accordingly, the principal object of the invention is to provide such a process and apparatus for growing void-tree sapphire fibers or the equivalent at a commercially desirable rate of growth, e.g. ten or more times faster than previously possible using the EFG process. Other objects will also be apparent from the following detailed description of the invention.